B. Reproduction of contrast 
Factors affecting the reproduction, in the negative, of the contrast at a boundary 
in the object include the following. 
(1) Colours of tbe object, colour sensitivity of tbe film, and type of filter used. 
Apart from their use to reduce the effect of haze, referred to below, film and 
filter combinations may also be used to enhance the contrast between certain colours 
in the object, while inevitably reducing the contrast between others. Any change in 
film and filter will therefore often involve a change in the interpretation of relative tone 
values. Black spruce may appear darker than jack pine in one case and lighter in 
another. 
(2) Haze 
The light from the sun scattered by the haze in the direction of the lens adds 
exposure to all parts of the negative and therefore reduces the contrast in the negative 
everywhere. This effect may be modified by the use of a filter that absorbs some, or 
most, of the light scattered by the haze. In this case, the object is photographed by the 
light which is not absorbed by the filter, but only to the extent to which the film is 
sensitive to that light. 
Common examples of this practice are, the use of a “minus blue” filter with panchro- 
matic film, and the occasional use of narrower band filters with films that are sensitive 
into the infra-red. Such changes involve a change in the interpretation of relative 
tone-rendering, and the remedy may be far from complete. 
(3) The film used, its development, and the resulting “camma”. 
“Gamma”, the slope of the density log exposure curve for the time of development 
used, expresses the ratio of the contrast in the negative to the contrast in the object, 
as “seen” by the particular film through the particular filter and atmosphere. Gamma 
increases with the time of development from zero to a maximum (which depends on 
the film, exposure and developer). 
Once the photograph has been exposed, the time of development (for a given 
developer) determines the contrast and maximum density of the negative, and thereby 
its resolving power (to an extent that depends on the developer) and the speed of the 
film. 
As has been pointed out the resolving power is greatest at density in the vicinity 
of 0.5 and is generally within 10% of maximum between densities of 0.2 and 1.2. 
(See Fig. 1.) 
Therefore, when maximum resolving power is required it will be necessary to 
choose a gamma that will accommodate the object brightness range within these 
density limits, and to increase exposure to compensate for any reduction in film speed. 
Incidentally — and most fortunately — these shorter-density-range negatives can be 
more readily and better reproduced (in the positive) than the usual more *contrasty" 
negatives. 
C. Reproduction of shape 
The exaggeration of the vertical scale in normal vertical photography has some 
advantages—easier perception of low relief, greater accuracy in measurements of height, 
greater economy—with fewer photographs. But a constant effort on the part of the 
interpreter is necessary to translate the exaggeration and “see” the object as it is. 
When, or where, does a cube look most like a cube? We use “the minimum distance 
of distinct vision”, 10 in. or 25 cm as a basis for defining the magnifying power of a 
lens or microscope. Is it not also the natural basis for expressing relative depth 
perception? A cube looks most emphatically like a cube at the minimum distance of 
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